This invention relates to sulfonated copolyesters that can exhibit an improved rate of biodegradation more amenable to solid waste disposal. The invention also relates to methods of making and using the copolyesters.
The inadequate treatment of municipal solid waste deposited in landfills and the increasing addition of nondegradable materials, including plastics, to municipal solid waste streams are combining to drastically reduce the number of landfills available and to increase the costs of municipal solid waste disposal. While recycling of reusable components of the solid waste is desirable in many instances, the costs of recycling and the infrastructure required to recycle materials is sometimes prohibitive. In addition, there are some products, which do not easily fit into the framework of recycling. One alternative approach is the composting of non-recyclable solid wastexe2x80x94a recognized and growing method to reduce solid waste volume for landfilling. Products from the composted waste can be used to improve the fertility of fields and gardens. However, one of the limitations to marketing such composted product is the visible contamination by undegraded plastic, such as film or fiber fragments.
Polymer components are sought that are useful in disposable products and which are degraded into less contaminating forms under the conditions typically existing in waste composting processes. It is further desirable to provide disposable components, which will not only degrade aerobically/anaerobically in composting, but will continue to degrade in the soil or landfill.
Polyesters have been considered for biodegradable articles and end-uses in the past. These biodegradable polyesters can be characterized as belonging to three general classes; aliphatic polyesters (polyesters derived solely from aliphatic dicarboxylic acids), aliphatic-aromatic polyesters (polyesters derived from a mixture of aliphatic dicarboxylic acids and aromatic dicarboxylic acids), and sulfonated polyesters derived from a mixture of aliphatic dicarboxylic acids and aromatic dicarboxylic acids and, in addition, incorporating a sulfonated monomer, such as the salts of 5-sulfoisophthalic acid.
A shortcoming of the above mentioned polyesters is that they often do not provide a composition which combines both high temperature characteristics, which are required by many enduses, such as dual ovenable food trays and the like, with a high rate of biodegradation, as desired to avoid the filling of landfills. It has been generally found that the biodegradation rate of the polyester may be enhanced through the addition of greater amounts of aliphatic dicarboxylic acids. At the same time, it has been generally found that the incorporation of such aliphatic dicarboxylic acids into a polyester composition tends to degrade the thermal properties of the polyester composition, as measured through the glass transition temperature, (Tg).
Isosorbide has been incorporated as a monomer into aliphatic and aromatic polyesters. A recent review is found in Hans R. Kricheldorf, et. al., J. M. S.-Rev. Macromol. Chem. Phys., C37(4), pp. 599-631 (1997). However it is generally believed that secondary alcohols such as isosorbide have poor reactivity and are sensitive to acid-catalyzed reactions.
One skilled in the art was thus confronted by three distinct art areas; (i) the sulfonated aliphatic-aromatic polyesters, which suffered from relatively low thermal properties, such as glass transition temperatures; (ii) the aliphatic isosorbide polyester art, which suffered from low molecular weights and thermal properties; and (iii) the aromatic isosorbide polyester art, which suffered from a low biodegradation rate.
The present invention has surprisingly found that the sulfonated aliphatic-aromatic isosorbide copolyesters of the present invention combine good molecular weight and thermal properties with improved biodegradability.
The present invention provides a sulfonated copolyester comprising the polymerization product of:
(a) one or more aromatic dicarboxylic acids or an ester thereof,
(b) one or more aliphatic dicarboxylic acids or an ester thereof;
(c) one or more sulfonated compound; and
(d) isosorbide.
The sulfonated aliphatic-aromatic copolyesters which incorporate isosorbide of the present invention are found to often avoid many of the shortcomings found in the art. The polymers of the invention can provide a combination of a higher biodegradation rate with higher thermal properties than found in the art.